Seed crystal and method for preparing single crystal

ABSTRACT

There is provided a seed crystal having a strength that the seed crystal is not broken even when the heavy single crystal is produced. The seed crystal is a seed crystal used for producing a single crystal according to Czochralski method wherein a relation between a sectional area of the seed crystal S (mm 2 ) and a weight W (kg) of the single crystal to be pulled is represented by the formula: S&gt;W. The single crystal is produced using the seed crystal. Furthermore, the single crystal is pulled with conforming a sectional area S (mm 2 ) of a seed crystal and a weight W (kg) of a single crystal to be pulled to the formula: S&gt;W.

TECHNICAL FIELD

The present invention relates to an improvement of a seed crystal usedin production of a silicon single crystal in accordance with Czochralskimethod (CZ method).

BACKGROUND ART

Conventionally, there is known for production of a single crystal, apulling method called Czochralski method (CZ method) wherein a seedcrystal is immersed in a melt of polycrystal raw material and is thenpulled to grow a single crystal. In the method, a seed crystal held by aseed crystal holding jig needs to be strong enough to bear the weight ofa grown crystal.

Recently, a single crystal, for example, CZ silicon single crystal hasbeen getting heavier with increase in a diameter thereof. For such atendency of increase in weight of the crystal, there have been proposedan improvement in resistance under load of the apparatus, a necking witha large diameter, or a new method substituted for a necking method, amethod of holding a crystal mechanically or the like. However, strengthof the crystal itself is one of important factors in common.

There have been various propositions as for a seed crystal and a seedcrystal holding jig. For example, there is proposed in Japanese PatentPublication (kokoku) No. 5-36395 a seed crystal holding jig consistingof a body of a seed crystal holding jig with a taper, a piece forpressing a seed crystal and a ring for preventing the piece fromdropping off. The seed crystal holding jig holds a seed crystal having atapered cut to pull a single crystal. With such a structure, it ispossible to hold a seed crystal without lowering strength of the seedcrystal.

A seed crystal to be held by such a seed crystal holding jig isgenerally a prismatic seed crystal having a tapered cut 1 as shown inFIG. 2A, or a columnar seed crystal having a tapered cut 1 as shown inFIG. 2B. In order to pull the crystal without conducting necking, a seedcrystal having a sharp tip end as shown in FIG. 2C can be used in somecases (See Japanese Patent Application No. 9-17687).

However, even when a seed crystal is held by the above-mentioned seedcrystal holding jig, it is sometimes broken depending on a shape of aseed crystal or weight of a single crystal to be grown and pulled.Breakage of the seed crystal may lead to fall of the single crystalhaving a heavy weight in a melt in a crucible, which will be a seriousaccident, and is problematic. On the other hand, it is not desirable forproduction of a seed crystal and a necking process that a seed crystalis unnecessarily thick.

There has not been an appropriate standard for defining a shape such asa thickness of a seed crystal so far, and a seed crystal having a shapedefined experientially has been used for producing a single crystal.

DISCLOSURE OF INVENTION

The present invention has been accomplished in order to solve theabove-mentioned problems, and the main object of the present inventionis to provide a seed crystal having a sufficient strength so that it maybe surely prevented from being broken, even when a heavy single crystalis produced.

To achieve the above-mentioned object, the invention set forth in claimsof the present application relates to a seed crystal used for producinga single crystal according to Czochralski method wherein a relationbetween a sectional area of the seed crystal S (mm²) and a weight W (kg)of the single crystal to be pulled is represented by the formula: S>W.

As described above, when the seed crystal wherein a relation between asectional area of the seed crystal S (mm²) and a weight W (kg) of thesingle crystal to be pulled is represented by the formula: S>W is used,the seed crystal is never broken even in the case that a heavy singlecrystal is pulled, so that a silicon single crystal can be safelyproduced using a seed crystal having a minimum thickness.

In that case, a sectional area means a sectional area of a seed crystalat a section vertical to a pulling direction.

It is preferable that the above-mentioned seed crystal has a tapered cuton the side. In that case, the seed crystal can be held firmly, so thatit can be held even if the single crystal to be pulled is heavy.Furthermore, strength of the seed crystal is not lowered so much, andtherefore, there is no fear of breakage of the seed crystal.

In this case, it is preferable that an angle of the above-mentionedtapered cut is 3 to 15 degree. It is further preferable that a sectionalarea of the above-mentioned tapered cut is 25% or less of the sectionalarea S of a seed crystal.

If the angle is less than 3 degree, depth of the cut at which a seedcrystal is held is too small, so that the seed crystal is held weakly.If the angle is more than 15 degree, sectional area of the seed crystalat a position where the cut is formed is too small, so that strength ofthe seed crystal is lowered, and there is caused a problem of strengthof the seed crystal holding rig. In order to prevent strength of theseed crystal from being lowered as a result of too small sectional areaof the seed crystal at a position where the cut is formed, a sectionalarea of the tapered cut is preferably 25% or less of the sectional areaS of the seed crystal.

A sectional area of a cut means a maximum sectional area at a sectionvertical to the pulling direction of a portion removed from a seedcrystal by cutting or the like in order to form a tapered cut on theseed crystal.

The present invention also relates to a seed crystal used for producinga single crystal according to Czochralski method wherein a relationbetween a minimum sectional area A (mm²) of the seed crystal at a partwhere the seed crystal is held and a weight W (kg) of the single crystalto be pulled is represented by the formula: A>W.

As described above, if a relation between a minimum sectional area A(mm²) of the seed crystal at a part where the seed crystal is held and aweight W (kg) of a crystal to be pulled is represented by the formula:A>W, the seed crystal is never broken at a part where it is held, sothat a silicon single crystal can be safely pulled.

The minimum sectional area of the seed crystal at a part where the seedcrystal is held means a minimum sectional area of a section of the partwhere the seed crystal is held by a holding jig or the like, which isvertical to the pulling direction of the seed crystal.

The above-mentioned seed crystal of the present invention can besilicon. The above-mentioned seed crystal can sufficiently cope with arecent tendency of a CZ silicon single crystal to be larger in diameterand be heavier.

In the method for producing a single crystal wherein the above-mentionedseed crystal is used, there is no fear of breakage of the seed crystal,so that a heavy single crystal can be safely produced. Especially, inthe method wherein a single crystal is pulled according to Czochralskimethod with conforming a sectional area S (mm²) of a seed crystal and aweight W (kg) of a single crystal to be pulled to the formula: S>W, thesingle crystal can be safely pulled without breakage of the seedcrystal, even when the heavy single crystal is pulled.

As described above, the seed crystal of the present invention enablesthe single crystal to be safely pulled without breakage of the seedcrystal, in the case that it is pulled according to Czochralski method,even if the single crystal has a large diameter and is heavy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A-FIG. 1D are explanatory views of examples of the seed crystal ofthe present invention.

FIG. 1A is a schematic view of the prismatic seed crystal.

FIG. 1B shows an example wherein cuts are formed on two sides of theseed crystal.

FIG. 1C shows an example where a cut is formed at a corner of the seedcrystal.

FIG. 1D shows a conic seed crystal, which is held at a side thereof.

FIGS. 2A to 2C are explanatory views showing examples of conventionalseed crystals.

FIG. 2A is a schematic view of a prismatic seed crystal.

FIG. 2B is a schematic view of a columnar seed crystal.

FIG. 2C is a schematic view of a seed crystal having a sharp tip end.

FIG. 3 is a graph showing a relation between a length (mm) of a side ofa regular quadrangular prismatic seed crystal and a breaking point ofweight (kgf) of the seed crystal.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be described more in detail. However, theinvention is not limited thereto.

The inventors of the present invention have conducted experiments andmeasurement as for a shape of a seed crystal that has beenconventionally defined experientially, in order to find a surer basisfor determining the shape, especially thickness of the seed crystal.Namely, tensile test of the seed crystal and measurement were actuallyconducted.

The tensile test was conducted by holding a seed crystal as a samplewith a seed crystal holding jig, loading a tensile force thereto, anddetermining a load when the seed crystal as a sample is broken. As theseed crystal as a sample, a prismatic seed crystal having a tapered cutas shown in FIG. 2A was used, and the test was conducted with varying asectional area S of the seed crystal. As a result, it has been foundthat a relation between a tensile strength F (kgf) of the seed crystaland a sectional area S (mm²) of the seed crystal was represented by theformula (1):

F≈S×10  (1)

Accordingly, in the case that the weight of the crystal to be pulled isW (kg), if a sectional area S (mm²) of the seed crystal conforms to thefollowing formula (2), the seed crystal is never broken.

10S>W  (2)

However, the inventors of the present invention have studied todetermine the value, taking safety factor of 10 times intoconsideration. If such a large safety factor is taken intoconsideration, it is possible to prevent breakage of a seed crystalalmost completely. Namely, a single crystal can be pulled under thecondition wherein the following formula (3) is fulfilled, with using aseed crystal wherein the following formula (3) is fulfilled.

S>W  (3)

The inventors have further studied and found that the above-mentionedformula (3) representing a relation between a sectional area S of a seedcrystal and a w eight W of a single crystal to be pulled can be appliednot only to a prismatic seed crystal, but also to a columnar seedcrystal as shown in FIG. 2B, a seed crystal having a sharp tip end asshown in FIG. 2C, and a polyhedral seed crystal as well. Even when asingle crystal is pulled using such seed crystals, it is possible toprevent a seed crystal from being broken under the condition conformingto the formula (3).

A sectional area S (mm²) means a sectional area of the section verticalto a pulling direction, not only in the case that a seed crystal is inthe shape of prism as mainly used in the test, but also in the case thatit is in the above-mentioned shapes such as a column or the like. Forexample, if the shape of the crystal is a column having a diameter D(mm), a sectional area S is πD²/4 (mm²).

The embodiment of the present invention will be described below.However, the invention is not limited thereto.

FIG. 1A shows a n example of a seed crystal of the present invention.The sectional area S (mm²) of the prismatic seed crystal is defined toconform to the condition of the formula (3), so that the crystal can besafely pulled.

The sectional area of the seed crystal that enables a crystal to bepulled safely can be calculated from the weight of the single crystal tobe pulled and grown, according to the above-mentioned method. However,the seed crystal needs to have a cut for being held by a holding jig.The shape of the cut is preferably a tapered cut 1 formed on the side ofthe seed crystal for sure holding of the seed crystal. When a seedcrystal is held by inserting a metal pin into a hole formed in the seedcrystal crosswise, the strength of the seed crystal will be lowered.However, when the seed crystal is held at the surface of theabove-mentioned tapered cut, strength of the seed crystal is not loweredso much, so that the seed crystal can be surely held.

In FIG. 1A, the cut is formed on one side of the prismatic seed crystal.However, the cut 1 may be formed on two or more sides of the seedcrystal as shown in FIG. 1B. Alternatively, the cut 1 may be formed at acorner of the seed crystal as shown in FIG. 1C.

The angle 2 of the tapered cut is preferably 3 to 15 degree. If theangle is in such a range, a depth of the cut, namely a sectional area 3of the tapered cut is not so small that the seed crystal may be heldweakly, and a sectional area 4 of the seed crystal at a position wherethe cut is formed is not so small that the strength of the seed crystalmay be lowered. If the angle 2 of the cut is too large, the part withwhich the seed crystal holding jig is in contact will be so thin and solong that the strength against loading may be lowered, and the holdingjig may be broken by the load. However, if the angle is in theabove-mentioned range, such breakage of the holding jig is never caused.

The appropriate angle 2 of the cut is applied not only to the seedcrystals as shown in FIG. 1A, but also to the seed crystals as shown inFIG. 1B and FIG. 1C.

A sectional area 3 of the tapered cut is preferably 25% or lower of thesectional area S of the seed crystal, in order to prevent a sectionalarea 4 of the seed crystal at a part where a tapered cut is formed frombeing too small. When the ratio of the sectional area 3 of the cut tothe sectional area S of the seed crystal is in the above range, the seedcrystal can be held sufficiently firmly. Furthermore, decrease of thesectional area of the seed crystal due to formation of the cut does notmatter, since a sufficient safety factor is taken into consideration inthe formula (3) showing a condition of a sectional area S of the seedcrystal and a weight W of a single crystal.

In addition to the above-mentioned condition, there may be set acondition that a relation between the minimum sectional area A (mm²) ofa part at which a seed crystal is held and a weight W (kg) of a singlecrystal to be pulled is represented by the formula: A>W.

The formula: A>W is a formula wherein the minimum sectional area A of apart at which a seed crystal is held is substituted for the sectionalarea S of the seed crystal in the formula (3). If the values conform tothe relation, the single crystal can be pulled safely without breakageof the part at which the seed crystal is held by the holding jig or thelike.

For example, in the case that the side of the conic seed crystal is heldwith a holding jig as shown in FIG. 1D, if a minimum sectional area A ofthe part at which a seed crystal is held with a holding jig as shown inFIG. 1D and weight W of a single crystal to be pulled conform to theformula: A>W, the single crystal can be pulled safely.

Of course, the conditions of the seed crystal of the present inventioncan be applied not only to the seed crystals shown in FIG. 1A to FIG. 1Dbut also to a columnar seed crystal, a seed crystal having a sharp tipend, a polyhedral seed crystal or the like.

The material of the seed crystal of the present invention is notlimited, however, if the seed crystal is made of silicon, asemiconductor silicon single crystal which tends to have a largediameter and be heavy especially recently, can be safely produced by apulling method. In that case, the seed crystal does not need to beunnecessarily thick, so that necking or the like can be conducted easilyand a seed crystal can be produced easily. Furthermore, cost can beminimized.

When the single crystal is produced by using a seed crystal of thepresent invention, the heavy single crystal having a large diameter canbe produced safely. Furthermore, the method of the present inventionwherein a single crystal is pulled with conforming a sectional area S(mm²) of a seed crystal and a weight W (kg) of the single crystal to bepulled to the formula: S>W can be applied to any cases wherein a singlecrystal is pulled according to a Czochralski method, to secure anoperation for pulling a single crystal.

The embodiments of the present invention will be explained below withexamples. However, the present invention is not limited thereto.

EXAMPLE, COMPARATIVE EXAMPLE

Supposing production of a silicon single crystal having a diameter of200 mm and a weight of 100 kg by pulling a regular quadrangularprismatic silicon seed crystal shown in FIG. 1A according to aCzochralski method, tensile strength test of the seed crystal wasconducted. The tensile test was conducted by holding a seed crystal as asample with a seed crystal holding jig, loading a tensile force thereto,and determining a load when the seed crystal as a sample was broken. Thetensile strength test was conducted with varying length of a side of theregular quadrangular prismatic seed crystal as a sample, namely asectional area S of the seed crystal, and a breaking load (kgf) when theseed crystal was broken was determined. An angle of a tapered cut of theseed crystal was 8 degree, and a sectional area of the cut was 15% ofthe sectional area S of the seed crystal.

The results were shown in FIG. 3.

FIG. 3 shows a relation between a side length (mm) of the seed crystaland a breaking load (kgf) of the seed crystal. Black circle plots showactually measured values. A dashed line shows a breaking load of theseed crystal calculated using a formula (1) from the sectional area Scalculated from a side length of the seed crystal.

As shown in FIG. 3, it was found that actual measured values conformedto the formula (1). Accordingly, when a side length of a seed crystalthat a sectional area S (mm²) of the seed crystal is more than a weightW (kg) of the single crystal to be pulled, is more than 10 mm, tensilestrength of the seed crystal is more than 1000 kgf, which is 10 times ormore of the weight of the single crystal to be pulled, and therefore,the single crystal can be sufficiently safely pulled by using the seedcrystal having the sectional area.

A silicon single crystal having a diameter of 300 mm and a weight of 150kg was pulled and produced according to Czochralski method using theseed crystal having a side length of the seed crystal of 13 mm, namelythe seed crystal having a sectional area S of the seed crystal of 169mm², used as a sample in the above-mentioned test. 100 single crystalscould be safely produced by a pulling method without causing anybreakage of the seed crystal or the like.

The present invention is not limited to the above-described embodiment.The above-described embodiment is a mere example, and those having thesubstantially same structure as that described in the appended claimsand providing the similar action and effects are included in the scopeof the present invention.

Moreover, the czochralski method in the present invention include mczmethod (magnetic field applied Czochralski crystal growth method) inwhich magnetic field is applied when the single crystal is pulled.

What is claimed is:
 1. A seed crystal used for producing a singlecrystal according to Czochralski method wherein a relation between aminimum sectional area A (mm²) of a seed crystal at a part where theseed crystal is held and a weight W (kg) of the single crystal to bepulled is represented by the formula: A>W.
 2. The seed crystal accordingto claim 1 wherein a tapered cut is formed on the side of said seedcrystal.
 3. The seed crystal according to claim 2 wherein an angle ofsaid tapered cut is 3 to 15 degree.
 4. The seed crystal according toclaim 3 wherein a sectional area of said tapered cut is 25% or less of asectional area S of the seed crystal.
 5. A method for producing a singlecrystal according to Czochralski method comprising: forming a melt ofpolycrystal raw material; immersing the seed crystal according to claim4 into the melt of polycrystal raw material; and pulling the seedcrystal to grow the single crystal.
 6. A method for producing a singlecrystal according to Czochralski method comprising: forming a melt ofpolycrystal raw material; immersing the seed crystal according to claim3 into the melt of polycrystal raw material; and pulling the seedcrystal to grow the single crystal.
 7. The seed crystal according toclaim 2 wherein a sectional area of said tapered cut is 25% or less of asectional area S of the seed crystal.
 8. A method for producing a singlecrystal according to Czochralski method comprising: forming a melt ofpolycrystal raw material; immersing the seed crystal according to claim7 into the melt of polycrystal raw material; and pulling the seedcrystal to grow the single crystal.
 9. A method for producing a singlecrystal according to Czochralski method comprising: forming a melt ofpolycrystal raw material; immersing the seed crystal according to claim2 into the melt of polycrystal raw material; and pulling the seedcrystal to grow the single crystal.
 10. A method for producing a singlecrystal according to Czochralski method comprising: forming a melt ofpolycrystal raw material; immersing the seed crystal according to claim1 into the melt of polycrystal raw material; and pulling the seedcrystal to grow the single crystal.
 11. The seed crystal according toclaim 6 wherein said seed crystal is silicon.
 12. A method for producinga single crystal according to Czochralski method comprising: forming amelt of polycrystal raw material; immersing the seed crystal accordingto claim 11 into the melt of polycrystal raw material; and pulling theseed crystal to grow the single crystal.
 13. A method of pulling asingle crystal wherein a single crystal is pulled according toCzochralski method with conforming a minimum sectional area A (mm²) of aseed crystal at a part where the seed crystal is held and a weight W(kg) of a single crystal to be pulled to the formula A>W.